Allow repeated fling boosting for accelerated scrolling

Allow repeated fling boosting for accelerated scrolling

Scrolling long documents on Android is a laborious, sad affair.  Native Android
view's address this with a "flywheel" fling mechanism, whereby successive flings
in similar directions accumulative velocity. Add such a feature to
InputHandlerProxy, allowing compositor-driven flings on all platforms to
quickly accelerate to infinity and beyond.

Flings will accumulate if the following conditions are met:
  - The active and new flings target the same layer, are in a similar direction,
    and are both of sufficient velocity.
  - All scroll update events received during the active fling and preceding the
    new fling are in a similar direction and of sufficient velocity.

Note: This approach is not suitable to WebView; it relies heavily on the fling
accumulator living "close" to the fling driver.  It also does not affect
main-thread driven flings.

BUG=164116

Review URL: https://codereview.chromium.org/239433010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@268363 0039d316-1c4b-4281-b951-d872f2087c98